This invention relates to communication systems and more specifically to communication transmitters and receivers which are capable of switching their channel frequency bandwidth.
In many communication applications it is desirable to employ radio receivers and transmitters which utilize variable filter bandwidths. For example, indoor wireless telephones and wireless local area networks WLANs, require such variable filter bandwidths.
Typically, systems that employ variable filter bandwidths are designed to include a plurality of filters with different frequency characteristics. One filter or a group of filters are then electronically selected to process a communication signal by employing one or more switches that route the signal to the appropriate set of filters.
As will be explained in more detail in reference with FIG. 1, such filter switching arrangement may be implemented, for example, in frequency multiplication stage of a transmitter or a receiver system. Typically, a mixer is employed to shift the frequency range of an incoming signal. In the case of a receiver, the mixer is used to shift down a high frequency signal to a lower frequency range. A switch is configured to receive the output signal from the mixer and route this output signal to an appropriate filter.
Such filter switching arrangement may also be implemented, for example, in a signal amplification stage. Typically, two or more filters with different frequency characteristics are configured to receive an amplified voltage signal via a switch.
One disadvantage with this switching configuration is that the use of switches in combination with mixers or amplifiers introduces signal distortions that leads to higher error rates. Furthermore, the termination impedances of each filter require to be substantially the same and match with the termination impedance of the mixer or amplifier output stage. However, it is sometimes difficult to design filters with different frequency bandwidths that exhibit substantially the same termination impedance.
Thus, there is a need for a communication system that employs variable filter bandwidths and has substantially no distortion due to the use of switches for routing signals to various filters.
In accordance with an exemplary embodiment of the present invention, a communication system with variable filter bandwidth comprises a first mixer circuit configured to receive a communication signal and shift the frequency range of the communication signal to a first frequency range; a second mixer circuit configured to receive the communication signal and shift the frequency range of the communication signal to a second frequency range; an activation signal coupled to the first and second mixer circuit so as to activate one of the mixer circuits; a plurality of filter circuits each configured to receive a signal from a corresponding mixer circuit, when the corresponding mixer circuit is activated.
In accordance with another exemplary embodiment of the present invention, a communication system with variable filter bandwidth comprises a plurality of Gilbert cells configured to receive a communication signal and a multiplying signal; a plurality of filters having a prespecified bandwidth such that each of the filters is configured to receive a signal from a corresponding Gilbert cell. Furthermore, each Gilbert cell is configured to go to an active state when it receives an activation signal.